A agent is used in a low-temperature container to keep the temperature very low. An example of freezing agent is liquid helium. The container (e.g., cryostat for a superconducting magnet) is formed to prevent transfer of heat from the outside. But as it is not possible to completely shut off the heat transfer, the freezing agent is evaporated to keep the temperature within the container at the desired value. The evaporated freezing agent is discharged through a path, such as a tube, to the outside. It is necessary to measure the amount of the freezing agent that has been consumed in order to the freezing agent consumption characteristics.
A prior-art freezing agent consumption measurement system is shown in FIG. 1. As illustrated, a freezing agent, such as liquid helium 2, is accommodated in a low-temperature container 1. The evaporated freezing agent is discharged through a tube 4. A gas mass flow meter 5 is connected to the end of the tube 4 to measure the mass flow rate of the gas that is discharged. The results of the measurement, flow rate, is recorded by a recorder, e.g., a pen recorder, connected to the flow meter 5.
The outlet 5b of the gas flow meter 5 is open to the atmosphere, so the gas that flows through the tube 4 and the flow meter 5 is discharged to the atmosphere.
A problem associated with the prior art freezing agent consumption measurement system is that as the atmospheric pressure changes, the flow of the evaporated gas is affected. That is, the flow of the evaporated gas is varied even if the consumption, i.e., the rate of evaporation, of the freezing agent is constant, and the flow of the evaporated gas does not accurately correspond to the consumption of the freezing agent.